‘‘I am aware of the ongoing federal investigation into my activities, and I am doing my best to address the situation responsibly, cooperate with the investigators, accept responsibility for my mistakes,’’ he wrote.

In his letter, he said he hopes to be remembered for what he accomplished and touted his accomplishments.

The 47-year-old took office in 1995 in a special election. He said he brought nearly $1 billion in federal funds to the district helping to build new train stations, community centers, a water tower, emergency rooms and new jobs.

But his district's boundaries, demographics and fortunes have shifted, particularly within the past decade as the economic boom of the '90s gave way to the recession, mortgage meltdown and foreclosure crisis of recent years.

In 2010, 69 percent of 2nd District residents were black, according to the U.S. Census.

A year later, Illinois Democratic leaders drew new legislative boundaries, extending the district from Chicago’s South Side to Kankakee. The district is now home to about 712,000 people, 100,000 more than before the remap, according to data compiled by the Illinois House Democratic Caucus.

The district’s new boundaries include portions of Kankakee and Will counties.

And 54 percent of voting-age residents are black, according to the Democratic Caucus’ data. White residents now account for more than 33 percent of the district’s voting-age population.

While detailed socioeconomic data for the new 2nd District are not yet available, 2011 Census estimates for the district prior to the remap illustrate its struggles with poverty, unemployment and education.

The pre-remap estimates show:

Though 19 percent of 2nd District residents lived in poverty in 2000, that rate grew to nearly 22 percent by 2011.

Twenty percent of residents were unemployed in 2011, which was much higher than the national unemployment rate.

In 2000, the area’s unemployment rate stood at 13 percent.

The district also lags behind when it comes to education. Only 21 percent of the district’s residents had earned a bachelor’s degree or higher, compared to 28.5 percent nationwide.

Jackson was re-elected for the ninth time on Nov. 6 while he was still at the Mayo Clinic. He didn’t have much of a re-election campaign, but an unofficial tally of votes by The Chicago Reporter shows that he managed to capture 63 percent of the vote, handily beating his opponents, Republican challenger Brian Woodsworth and Independent Marcus Lewis. However, even in a landslide, the results were a potential sign of waning support for Jackson. In all of his previous re-election bids, he’d won at least 80 percent of the votes.

The special primary to replace Jackson has been scheduled for Feb. 26 and the general election is April 9.

An earlier version of this story stated that 69 percent of constituents within the district’s boundaries were black. The 69 percent figure actually represents the proportion of African Americans living in the district before its boundaries were changed in 2011.